Improving process inventory and quality control is a continuous and challenging endeavor that often includes the implementation of in-process bar code and other labeling systems. Fabrication of a bar code label that is able to survive high temperature processing conditions, e.g., 400 C or more, is particularly challenging. For example, a bar code label applied to a television picture tube, i.e., a cathode ray tube or CRT, must be readable with a scanner after exposure to temperatures greater than 400 C.
Green ceramic sheets, i.e., labels before they have been exposed to a high temperature, have been used as high temperature labeling material since at least as early as 1990. U.S. Pat. No. 4,971,858 (Yamano), U.S. Pat. No. 5,204,163 (Nakatsuka) and U.S. Pat. No. 5,209,796 (Sakuramoto) describe green ceramic sheets composed of a base layer and a layer of pressure-sensitive adhesive (PSA) protected with a release film. The base layer is fabricated from glass frit and one or more binder resins, e.g., a silicone resin, and it carries an image comprising a high temperature pigment. Labels made from such a sheet are applied to an object destined for high temperature processing in which the binder resin decomposes, the glass frit melts, and pigment adheres to the object.
U.S. Pat. No. 5,506,016 (Onodera), U.S. Pat. No. 5,578,365 (Kume), and U.S. Pat. No. 6,416,845 (Kume) also describe a green ceramic sheet formed from a base layer and a PSA layer but rather than using glass frit in the base layer, a silicone resin and/or rubber are included to hold the pigments on the object during and after the high temperature process. Yet another green ceramic sheet is described in U.S. Pat. No. 5,780,142 (Kume) as having a base layer of acrylic resin and glass frit, and an overlying silicone resin layer bearing an inked image.
One drawback of currently available high temperature labels is the use of glass frits due to their toxicity, particularly low melting point glass frits, which contain lead. Another drawback of current high temperature label materials is the inclusion of a silicone resin as a binder or ink-receptive layer, which can result in poor printability and, accordingly, narrow the selection of ribbons.